1. Field of the Invention
The present invention pertains to contact lenses and in particular to multi-focal lenses for simultaneously imaging objects at multiple distances (e.g., near, intermediate or far) in a wearer's eye.
2. Description of Related Art
Although the popularity of eyeglasses for use in mono vision correction remains strong, considerable attention has been directed to the use of contact lenses in correcting the more difficult vision problems of presbyopic patients and patients suffering from pathological conditions. The fitting of presbyopic contact lenses has become a specialized field, requiring fitting techniques and highly developed skills not required in other types of contact lens practice.
A contact lens can be fitted so as to remain centered on a patient's cornea or pupil (a so-called "central fit"), or the contact lens can be positioned off-center with respect to the wearer's eye. It is widely recognized that a contact lens is rarely stable in a person's eye and the shifting of the lens with respect to the pupil due to lid and eye movement, for example, has long been recognized.
A so-called "alternating vision" fitting practice relies on a shift in the wearer's eye to move the near and the distant correction portions of the lens with respect to the wearer's pupil. Even with fitting practices which rely on a relatively constant positioning of the contact lens about the wearer's eye, movement of the lens with respect to the pupil must be anticipated. One example of this latter type of fitting practice is the so-called "simultaneous vision" fit in which nearby and distant objects are simultaneously processed by the user, requiring mental differentiation of the resulting multiple images.
One approach for providing a precise fitting is to shape the contact lenses with a "close fit", closely conforming the curvature of the lens to the curvature of the patient's eye, so that movement of the lens about the eye is restricted. If the curvature of the lens is increased beyond this point, stability is attained at the expense of patient discomfort, and recent studies have indicated problems of inadequate circulation and cleansing and wetting action of tears associated with tighter-fitting lenses.
One type of vision correction that has been attempted over the years is commonly referred to as "pinhole lenses". These types of lenses have been considered for correction of presbyopia but have been rejected as reducing to an impractical extent, both the field of view and the brightness of the retinal image. These types of lenses are generally regarded as unsuitable, despite costly enhancements such as multi-range features where a central pinhole is surrounded by several radial lines. Although the field of view is improved somewhat, reductions in retinal image brightness are severe, and it is difficult for patients to read in many situations. As is well understood in the field of optics, enlarging the pinhole size to increase image brightness leads to a reduction in image sharpness, or resolution.
Prior art aperture-type pinhole lenses are made from black plastic stock so as to provide a dense black periphery around the pinhole, which is formed by drilling and subsequently filling the lens with clear plastic. Even when the pinhole lens is fitted so as to move with the eye and so as to be very closely positioned with respect to the entrance pupil, the field of view is still greatly restricted compared to other types of presbyopic vision correction. Construction costs are higher than other vision correction lenses, and the resulting performance has been disappointing, especially for normal vision patients who complain of excessive loss of light and non-specific types of visual disturbance. Cosmetic appearance presented by this type of pinhole contact lens has been found to be unacceptable for many patients.
In the construction of prior non-aperture pinhole lenses, the majority of the contact lens is made opaque with only the central pinhole aperture transmitting light. Accordingly, the cosmetic appearance presented by the pinhole contact lens is unacceptable for many patients. As mentioned above, radial lines are sometimes added to the pinhole of an opaque lens to improve peripheral vision. A series of non-opaque radial spokes are provided, emanating from the pinhole. These types of contact lenses present an unacceptable cosmetic appearance. According to work performed in association with the British Optical Association substantial disadvantages of pinhole lenses have been quantified. The diameter of the pinhole aperture is chosen between one and two millimeters according to the near addition. The higher the near addition, the smaller the aperture needs to be in order to provide the patient with acceptable vision correction. It is estimated that a 1.00 millimeter aperture is the equivalent of a very dark tinted spectacle lens. Further, the reduced field of view is estimated to be as little as fifteen degrees depending upon the distance of the lens from the eye's entrance pupil (interior chamber).
Although the pinhole contact lens provides improvement to correct refractive irregularities, especially for patients having irregular eye surfaces, the resulting reduced illumination renders the use of these types of lenses unacceptable in dim light and especially at night.
Because of the drawbacks reported by large numbers of healthy presbyopic patients, other types of vision correction are usually employed, and pinhole lenses are regarded as providing value for patients having low vision due, for example, to distorted pupils, scarred corneas or other impairment to the eye's optical system.